The present invention relates to a phase synchronizing circuit for recovering a synchronized carrier wave from a multi-phase PSK (phase-shift-keying)-modulated carrier wave for demodulation, and more particularly, to a phase synchronizing circuit composed of base-band signal processing circuits.
Usually, a phase synchronizing circuit necessary for demodulation uses an inverse modulation system, a remodulation system, a frequency multiplication system, etc. in which signal processing is carried out in a high frequency band (or a carrier wave-band), but recently phase synchronizing circuits relying upon base-band signal processing are being investigated. The base-band signal processing circuits are advantageous in that they may be inexpensively manufactured. That is, since the frequency to be processed is in the base-band frequency region, realization of the circuit arrangement is easy and of low cost. In addition, since a phase error signal is derived from a deviation from an optimum point in a demodulated eye-pattern signal, an optimum demodulated pattern signal can be obtained.
On the other hand, base-band signal processing circuits are somewhat inaccurate. Thus, a phase error signal corresponding to the phase difference between the demodulated carrier wave and the recovered carrier wave is produced in the phase synchronizing circuit in which signal processing is carried out in a carrier waveband. Accordingly, if a quadruple frequency-multiplier output is obtained as a phase error signal (for example, in the case of a 4-phase PSK-modulated carrier wave) then even if its phase stable point should deviate from that of the demodulated output, phase synchronization could be accomplished by varying an electrical length in a carrier wave-band. In the case of a phase synchronizing circuit relying upon base-band signal processing, since a phase error signal is produced by synthesizing amplitudes of demodulated eye-pattern signals, phase synchronization by varying an electrical length is impossible. In the heretofore known methods, only .+-.(.pi./4) phase-shifters (in which phase-shift is accomplished by performing addition or subtraction between orthogonal demodulated signals) have been used. Consequently, there is a first disadvantage that only when a quadruple frequency-multiplied output is obtained as a phase error signal and also a phase stable point of the quadruple frequency-multiplied output and that of a demodulated signal coincide with each other, the phase synchronizing circuit would be practically useful, but if the phase stable point is deviated outside of .+-.(.pi./4), then the phase synchronizing circuit would be practically useless.
Further, a D.C. component is usually included in the phase error signal obtained by synthesizing the demodulated signals. Consequently, there is a second disadvantage that if level variation should occur in an input signal, the D.C. value of the phase stable point in the phase error signal would be varied, resulting in the variation of a phase-locking center frequency and in a stationary phase error.